1. Field of the Invention
The present invention relates to an image output system which prints based on print data described in a page description language.
2. Description of the Related Art
An image output apparatus (also called an image forming apparatus) such as a page printer which executes an image process in accordance with externally input print data performs a rendering instruction or device control on the basis of a page description language (to be referred to as a PDL hereinafter). The page description language is described in a predetermined format (syntax).
For example, there are various application software programs which run on an operating system (OS) such as MS-Windows® running on a personal computer and create various electronic documents. When the application software is to print, a printer driver running on the OS converts a rendering instruction output from the application software into PDL print data interpretable by an image output apparatus (printer). The printer driver outputs the print data to the image output apparatus via a predetermined interface. The printer driver and image output apparatus construct an image processing system. The printer driver converts a print request from application software into PDL print data of each page in accordance with a predetermined syntax.
A conventional page printer or the like analyzes received PDL print data in accordance with the PDL format, and generates an intermediate rendering object (to be referred to as a display list or simply a DL hereinafter). The page printer generates a bitmap image, i.e., page image representing the contents of each page on the basis of the intermediate rendering object. The conventional PDL contains various instructions for issuing a rendering instruction for each page. For example, the PDL includes a character print instruction, various graphic rendering instructions (e.g., line rendering instruction), an image rendering instruction, rendering logic (ROP designation such as AND or OR), an instruction to designate a clipping area, a macro instruction to group, store, and call a plurality of instructions, and an instruction group to set various print environments.
Every time generating a page image, the page printer outputs to the printer engine a /PRNT signal for designating the start of forming the page image. Subsequently to the /PRNT signal, the page printer transmits a video signal corresponding to the page image to the printer engine, and forms an image on a sheet surface by a known electrophotographic process, outputting the page image.
The image forming apparatus having this arrangement does not always complete analysis of a PDL instruction in received print data within a short time. If analysis of the PDL instruction takes a long time, the printer engine must wait until the page image is prepared. For example, paper feed starts before the end of an image analysis process for a given page when the end timing of the image analysis process is earlier than the start timing of sending the video signal of the page. If the end timing of the image analysis process is later than the start timing of sending the video signal, paper feed starts after the image analysis process ends (see Japanese Patent Laid-Open No. 7-256935). Alternatively, a host computer predicts and calculates in advance the image expansion process time when the engine can output print data of the start page, adds the calculated process time to print data, and transfers the print data (see Japanese Patent Laid-Open No. 2000-207146). The printer starts the engine after the delay of the added process time.
In the above-described prior arts, however, the PDL analysis process requires a long time in accordance with the degree of complexity of a rendering process designated by application software, and the printer engine cannot fully enhance its performance.
More specifically, if the printer engine receives PDL data having a heavy process load, the PDL data analysis time, i.e., DL generation time exceeds the upper limit which can maintain the throughput of the printer engine. The upper limit which can maintain the throughput of the printer engine is the formation time necessary to form, e.g., an image on one sheet medium. If the DL generation time exceeds this formation time, the printer engine suffers an idle time. In general, the printer engine temporarily stops upon generation of its idle time. In this case, the printer engine performs a post-rotation process. The post-rotation process is a process until the printer engine temporarily ends image formation and temporarily stops. Once the printer engine stops, it must execute a pre-rotation process to activate the printer engine so as to form an image again. The time taken for the post- and pre-rotation processes is the down time of the printer engine. For example, for a printer engine having a throughput of 30 ppm (pages/min), a page image of each page must be prepared within 2 sec in order to maintain the highest throughput. This is because a page image of a given page must be formed while printing a page immediately previous to the given page. If PDL analysis takes 2 sec or more, the printer engine temporarily stops. For this reason, the printer engine stops and restarts again and again while executing a print job of high-resolution printing in which pages with a heavy PDL process load continue.
According to Japanese Patent Laid-Open No. 7-256935, the feed timing is adjusted, but the down time of the printer engine cannot be prevented. According to Japanese Patent Laid-Open No. 2000-207146, the start timing of image formation can be delayed for only the start page. However, the down time of the printer engine between pages cannot be avoided.